A method for producing a silicon ingot is known from DE 10 2005 013 410 B4. As the crystal structure of a silicon ingot of this type has a substantial effect on the properties of the components subsequently produced therefrom, there is a continuous need to improve a method of this type.
The invention is therefore based on the object of improving a method for producing a silicon ingot.
This object is achieved by a method for producing a silicon ingot comprising the following steps of providing a container to receive a silicon melt, providing a temperature control device to control the temperature of the silicon melt in the container, arranging raw material in the container comprising silicon and at least one nucleation agent to assist a heterogeneous nucleation in the silicon melt, and control of the temperature in the container in such a way that the raw material is present, during a specific method portion, as silicon melt in the container, which is solidified in a directed manner during a subsequent method portion, wherein the nucleation agent comprises nanoscale particles.
The core of the invention is to add a nucleation agent to the silicon for producing a silicon ingot to assist a heterogeneous nucleation in the silicon melt. Owing to the heterogeneous nucleation a grain refinement occurs in the volume of the silicon ingot, the grain refinement being able to occur in an increased extent in the lower volume close to the base. The heterogeneous nucleation is furthermore a prerequisite for a growth-induced grain selection, in which crystallites rich in defects are overgrown by those low in defects. As a result, the defect density, in particular the dislocation density, is significantly reduced in the silicon crystal.
The structure close to the base firstly consists both of crystallites that are rich in defects as well as those free of defects. It was recognised according to the invention that the crystallites rich in defects are overgrown by crystallites free of defects with suitable control of the crystallisation process. The method according to the invention therefore leads to the production of a silicon ingot with a particularly low defect density over the entire ingot height. In particular, a dislocation multiplication is suppressed by the method according to the invention, so that theoretically ingots of any height can be produced.
According to the invention, it was further recognised that it is advantageous for the configuration of a crystal structure low in defects if the crystallites that are rich in defects have a large number of neighbours low in defects. This can be achieved in particular, in that the nucleation agent comprises nanoscale particles. Nanoscale particles are taken to mean here particles, the diameters of which are in the range from 10 nm to 10 μm, in particular in the range from 10 nm to 1 μm, in particular in the range from 10 nm to 500 nm, in particular in the range from 20 nm to 100 nm. In particular, the mean grain size of the particles is less than 1 μm, in particular less than 200 nm.
The nanoscale particles can be mixed with the raw material as a solid material, called an addition below. Alternatively, particles can also be advantageously formed by melt synthesis in the silicon melt, called precipitation below. The effect is utilised here that with an adequate concentration of the nucleation agent components in the silicon melt, a supersaturation precipitation of the nucleation agents occurs during the solidification or further cooling. Advantageously, the size of the nucleation agent particles precipitated here can be controlled in a targeted manner by a targeted conduct of the process, in particular by suitable control of the temperature course during the solidification of the silicon melt.
The chemical nature of the particles used as nucleation agents primarily consist of the quaternary system Si—O—N—C. At least 90% by weight of the particles preferably originate from this substance system.
The composition can be defined by Si1CxOyNz, wherein there applies: X: 0-2, Y: 0-3, Z: 0-4, wherein X, Y, Z does not necessarily have to be integral. In particular, the known stoichiometric compounds SiC, SiO2, Si3N4, Si2N2O are contained in this quaternary system.
The particles preferably form getter centres for movable metal atoms in the silicon ingot. This effect is based on the fact that movable metal atoms in the silicon ingot have a greater affinity to specific precipitations. The nucleation agent particles can, in particular, hinder the diffusion of harmful metal atoms into the ingot.
Addition variant: in an advantageous embodiment, the finely dispersed nucleation agent of the Si1CxOyNz system is added to the raw material during the filling of the mould. It was surprisingly established that the particles survive the melt process although at least individual ones of the dissolved O-, C- and/or N-concentrations are below the equilibrium concentrations known in the literature. These particles develop their nucleation agent effect during the subsequent solidification process.
Precipitation variant: in a further advantageous embodiment, the nucleation agent, in particular the nucleus components, during one method portion, is/are completely, or at least partially, dissolved in the silicon melt. This allows a particularly uniform distribution of the precipitating nucleation agent.
The precipitation agent is preferably arranged in the crucible in such a way that it, in particular at least one, preferably more or all its components in the dissolved state, has/have a concentration in the silicon melt, which is greater, at least in regions, than a saturation concentration of the respective component or the respective components in the silicon melt. This leads to a supersaturation precipitation of the nucleation agent occurring while the melt is being cooled. According to the invention, the precipitation of the nucleation agent substantially occurs before the solidification of the melt.
By suitable control of the temperature field in the melt crucible, in particular by suitable control of the convection in the crucible, the supersaturation precipitation can be limited to specific regions in the melt, in particular to the region close to the crucible base. Moreover, the size of the precipitation can be influenced by controlling the temperature field and the temperature course in the crucible. It was determined in this regard that a stronger convection leads to a smaller size of the precipitated particles. The convection in the crucible is therefore advantageously controlled in a targeted manner at least during a predetermined method portion, in particular before the beginning of the solidification of the silicon melt by means of the temperature control device.
The nucleation agent can preferably be arranged in the container in such a way that a concentration gradient thereof occurs in the silicon melt. The concentration of the nucleation agent, in particular in a volume region close to the base, is higher here than in the remainder of the silicon melt. As a result, the crystallites rich in defects can be concentrated to the region of the silicon ingot close to the base. This region, i.e. the ingot base, is separated in any case from the silicon ingot in the further process. The remainder of the ingot has a correspondingly improved crystal structure.
The nucleation agent particles, at least in a specific region in the silicon melt, in particular in the region of the crucible close to the base, preferably have a density (particles per volume unit) of at least 102 cm−3, in particular at least 104 cm−3, in particular at least 107 cm−3.
A further object of the invention consists in providing an improved silicon ingot.
This object is achieved by a silicon ingot comprising a longitudinal axis, a first end in the direction of the longitudinal axis, a second end in the direction of the longitudinal axis, a length in the direction of the longitudinal axis, a multi-crystalline structure and a grain density, which, in the region of the first end, is at least 400 dm−2, in particular at least 600 dm−2, in particular at least 800 dm−2.
The core of the invention consists in providing a silicon ingot with a high grain density at the end, in particular on the base side. The advantages of the ingot according to the invention emerge from those which were already described in relation to the production method thereof
A further object of the invention consists in providing a silicon wafer with a small defect density and low metal content. This object is achieved by a silicon wafer made of multi-crystalline silicon with a wafer surface and with particles, wherein at least 90% of the particles have a diameter of at most 1 μm, and the particles have a fraction of a compound of silicon and at least one of the elements selected from the group of carbon, oxygen and nitrogen.
Further advantages, features and details of the invention emerge from the description of embodiments.